Group A Streptococcus (GAS) is a Gram-positive bacterium that causes several diseases in humans, including pharyngitis and/or tonsillitis, skin infections (impetigo, erysipelis, and other forms of pyoderma), acute rheumatic fever (ARF), scarlet fever, poststreptococcal glomerulonephritis, and a toxic-shock-like syndrome. On a global basis, ARF is the most common cause of preventable pediatric heart disease. There are 25-35 million cases of streptococcal pharyngitis per year in the United States, responsible for about 1-2 billion dollars per year in direct health care costs. Beginning in the mid-1980s, an intercontinental increase in streptococcal disease frequency and severity occurred for unknown reasons. The continued great morbidity and mortality caused by GAS in developing nations, the significant health care financial burden attributable to GAS in the United States, and increasing levels of antibiotic resistance in this pathogen highlight the need for a fuller understanding of the molecular pathogenesis of streptococcal infection. Moreover, the increase in disease frequency and severity stress the lack of an efficacious vaccine, despite the repeated inclusion of GAS in lists of important human pathogens for which vaccines are needed. We are conducting a detailed analysis of the molecular pathogenomics of GAS, with the goal of understanding the molecular basis of epidemic behavior, strain emergence, inter-strain variation in virulence, and host immune response. A related goal is the development of new therapeutics, including a vaccine. This project involves genome sequencing, DNA-DNA microarray analysis, expression microarray analysis, proteomics, animal model studies, in vitro and in vivo studies of novel proven or putative virulence factors, and conventional molecular pathogenesis strategies with isogenic strains. We have deployed a fully-integrated strategy that focuses on molecular events occurring at the interface between GAS and the infected human host or relevant animal host model. Efforts are also directed to analysis of genetically representative strains recovered from patients with defined clinical syndromes. Toward this end, we have used expression array analysis to discover that GAS differentially regulates expression of 9% of chromosomal genes in response to physiologically significant changes in growth temperature. Expression array analysis also has been used to reveal a complex virulence gene regulatory network controlled by a two-component system. We have successfully sequenced the genomes of a strain of serotype M18 cultured from a patient with acute rheumatic fever, and a strain of serotype M3 from a patient with necrotizing fasciitis. DNA-DNA microarray analysis of serotype M18 strains has provided new information about the origin of ARF epidemics.